"High water" Memory fragmentation still a thing?

mm0fmf none at mailinator.com
Sun Oct 5 21:23:42 CEST 2014


On 04/10/2014 02:02, Steven D'Aprano wrote:
> Way back in the mid 1980s, Apple Macintoshes used a memory manager which
> could move memory around.

But the memory manager didn't return a pointer to memory the way malloc 
does. It returned a pointer to the pointer and you had to double 
dereference it to get the heap address (ISTR, 30 years ago now). The 
advantage being the memory manager could shuffle the memory about and 
update the pointers. Your pointer to a pointer would still point to the 
same block after a shuffle. Of course you couldn't hold on to a partial 
dereference across system calls... can you guess why? :-)

Linux has (had) a allocation scheme where blocks came from different 
sized areas depending on the size requested. So all requests below 4k 
came from one heap area, and so on for 16k, 64k 256k, 1M etc. Meaning 
that code requesting a freeing up small amounts fragged the small 
allocation zone and so a big allocation would die due to fragmentation 
of small amounts. That was in kernel 2.4 days, sorry I'm off the 
bleeding edge now with how the allocator works in modern kernels.

Andy



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